John Gill Bible Commentary

Now, therefore, write ye this song for you,.... Which was now dictated by the Lord, and given to Moses and Joshua to write, which is recorded in Deu 32:1, and teach it the children of Israel; teach them by it, instructing them in the meaning of it: thus it was usual in ancient times to write things in verses, that they might be the more pleasingly attended to and regarded, and be longer retained in memory; and especially this practice was used with children, and still is: put it in their mouths; oblige them to get it by heart, or lay it up in their memories, and repeat it frequently, that it may be familiar to them, and not be forgotten by them: that this song may be a witness for me against the children of Israel; when in times to come they shall call to mind how in this song they were cautioned against such and such sins, and what they were threatened with should befall them on account of them, and how all things have come to pass exactly as foretold in it; which would be a testimony for God of his goodness to them, of his tender care of them, and concern for them, in the previous cautions he gave them; and of his foreknowledge of future events; and a testimony against them for their ingratitude and other sins.